offsetof

offsetof is a macro defined in the C standard library header stddef.h. It yields the offset, in bytes, of a named member within a structure or union, relative to the beginning of that type. The result has type size_t and is intended for compile-time calculation of a member’s position in memory. A typical implementation is the expression ((size_t) &(((type *)0)->member)), which forms the address of the member as if the object at address zero were laid out with that member, then casts it to size_t.

Usage example: struct S { int a; char b; }; size_t off_b = offsetof(struct S, b); // often 4 on many ABIs. Offsets computed by offsetof are used in low-level code for tasks such as serialization, memory mapping, and generic container macros that operate on embedded structures.

Portability and limitations: The macro is defined for standard-layout types, and the value is a compile-time constant in conforming implementations. The offset for bit-fields or unusual layouts is implementation-defined or not meaningful, so offsetof should be used with caution. Different compilers or architectures may produce different offsets due to alignment and padding. In practice, offsetof is a tool for internal memory-layout calculations rather than a general reflection facility. See also container_of, a common pattern that uses offsetof to recover a pointer to a containing structure from a pointer to one of its members.